The present invention relates to desalination, and, more particularly, to an apparatus and a method utilizing solar energy in the desalination process.
Distillation devices are used to purify volatile substances, and have found application in the chemical and petroleum industries. Recently, such devices have been used in desalination processes wherein salts are precipitated out of saline water to produce pure water. Recently, there has been an interest in applying distillation processes to desalinating large quantities of water. This interest has been sharpened due to droughts in vast land areas which are, however, located near great bodies of salt water. An apparatus and a process which efficiently desalinates large amounts of water could find wide applications in such drought ridden areas, as well as in densely populated areas which, at present, appear to have adequate water supplies.
Most known distillation devices suffer a common drawback of inefficiency, and hence are not suitable for use in such large scale applications. This inefficiency results in either low outputs of pure water or in extremely high power requirements. Therefore, known devices are either incapable of supplying the quantity of pure water reqiired, or require such high power inputs as to result in unacceptable environmental pollution.
Some known devices have somewhat remedied the inefficiency of the distillation process by utilizing solar energy as a means of augmenting the power input. The solar energy is commonly utilized during the vaporization step in the distillation process. However, these devices vaporize the water at atmospheric conditions, and the heat input required to vaporize water at atmospheric conditions is quite large. Therefore, the output of such devices is quite limited. These devices must be either unreasonably large or require large solar energy transfer devices. Thus, there are known devices which utilize solar energy in the vaporization step which employ solar grids covering several acres of area, and large amounts of land area are consumed by the solar heating means itself.
A further drawback of known devices results because the vaporization and condensation steps are carried out in a common chamber. Thus, the distillate often becomes recontaminated due to its proximity with the solid residue originally removed therefrom. It is for this reason that many of the known distillation devices provide recirculation systems for recycling the distillate in an attempt to assure total removal of substantially all of the solid contimanants. However, because of the above-mentioned proximity of the distillate and the solid residue, known processes are somewhat self-defeating. Because of the recycling, procedure, the net output of known devices is further limited and total separation, along with efficiency, are goals which, in many ways, are exclusive of each other.
As is well known, as the pressure of a system is decreased, the saturation temperature of water in that system also decreases. For example, at atmospheric pressure, the saturation temperature of water is 212.degree. F., at 10 psia it is 193.21.degree. F., at 5 psia it is 162.24.degree. F., at 1 psia it is 101.74.degree. F., and at 0.0886 psia it is 32.02.degree. F. (the triple point). Therefore, the power required to raise water to the saturation temperature is decreased as the pressure of the system is decreased. This fact is utilized by some known distillation systems. However, due to the nature of the elements used by these devices to produce the reduced pressure environment, only a small amount of liquid can be treated at one time. Furthermore, these devices do not produce a continuous flow through the distillation apparatus. Thus, the saline water is flooded into a chamber, the chamber is then closed off, a reduced pressure condition is then produced in the chamber, and then heat is input into the saline water to vaporize it. Therefore, inflow into the distillation apparatus is interrupted while the vaporization process is occurring, and the production of pure water of the system is therefore limited. Such a system is therefore not suitable for desalinating extremely large quantities of water. Furthermore, due to the nature of the pressure reduction step, the vaporization step is carried out in a chamber which communicates with the condensation chamber in such a way that the distillate is exposed to the solid residue, therefore producing the above-discussed possibility of recontaminating the distillate and thereby reducing the effectiveness of these systems.
In the device embodying the present invention, water is heated utilizing solar energy and then transferred to a vacuum chamber where it is vaporized and the water vapor separated from the solid residue in a continuous flow process. The water vapor is then transferred to a separate condensing chamber where it is condensed to form pure distilled water.